A typical method of treating polluted water such as waste water is a method of separating sludge by flocculation precipitation or floatation separation which contains generating floc by adding an inorganic flocculant and then adding a polymer flocculant to the polluted water. A part of treated water by such a method may be effectively used; however, most of the treated water is not effectively used because there are problems in water quality and stability of treatment. Usually, the treated water is discharged into a river or the like. If polluted water is subjected to an advance treatment including filtrations such as microfiltration (MF), ultrafiltration (UF); and sand filtration, it is anticipated to improve water quality and obtain available water. However, since filtration speed is low and a filtration layer tends to clog severely due to a suspended material included in the polluted water, it is difficult to realize.
An example of a method of purifying water for obtaining purified water which is able to be used as tap water contains a sand filtration step or a membrane filtration after treating with a flocculation precipitation method by adding polyaluminum chloride (PAC) which is an inorganic flocculant to raw water. However, when the raw water has low water quality and includes many suspended solids, a large amount of PAC is required and the suspended solids remain in the treated water, as a result, efficiency of filtration tends to decrease markedly. Therefore, it is difficult to purify the polluted water by this method.
Recently, in the field of water purification techniques, a method of purifying water by filtering and separating treated water flocculated by adding a polymer flocculant to raw water has been investigated in detail, and the method has been reported in the following Non-patent Document 1.
Non-patent Document 1 points out adverse effects such that use of a polymer flocculant causes improvement of the treated water quality, but increase of speed of head losshead loss in the sand filtration though the turbidity of the treated water is decreased and water quality is improved. In particular, there are large adverse effects to the head loss when the condition of flocculation is no good or the polymer flocculant is excessively added. Non-patent Documents 1 indicates that it is difficult to control use conditions such as an appropriate dosage, flocculation condition, injection position, and the like of the polymer flocculant.
Patent Document 1 discloses a method of solid-liquid separation by adding an amphoteric polymer flocculant, as a method of flocculation treatment of sewage. The invention of Patent Document 1 aims to improve the dewaterbility of flocculated sludge, and does not disclose the filtration property of flocculation-treated water.
[Non-patent Document 1] “Guideline of Development Research of Water Purification Techniques for Reduction of Environmental Impact (e-Water)”, Japan Water Research Center, August 2005, p. 178 and 201-202
[Patent Document 1] Japanese Unexamined Patent Application, First Publication No. 7-328644